Assessing & recognising the soil carbon benefits of organic farming

Transcription

1 Assessing & recognising the soil carbon benefits of organic farming Gundula Azeez, Soil Association Presentation at Enita Clermont-Fd, France, April 2008 Soil Carbon - introduction Soil carbon is a very important subject for this conference: one of the main drivers of climate change lower levels will exacerbate impacts of climate change a main & characteristic difference of organic farming probably one of the main climate change benefits of organic farming but soil carbon is omitted from most assessments of the Global Warming Potential of organic farming. 1

2 Soil carbon losses - the causes? Agriculture has caused considerable historic soil carbon losses, and research indicates that losses are continuing together, the historic & current losses show the potential for soil carbon sequestration by farming these losses are being attributed to specific causes but we propose that: the adoption of industrial farming practices, especially inorganic fertiliser, is actually a main cause. Soil carbon losses - historic attributed to the ploughing up of permanent grassland in UK, this occurred from the 1940s+ (King et al, 2005) but most dramatic losses were mid-1970s to mid- 1980s traditional practices to maintain soil organic matter were abandoned: farmyard manure etc. (MAFF, 1970) these changes were all enabled by inorganic N fertiliser may have reduced micro-organisms that create humus so, inorganic fertiliser may be a major cause. 2

3 Soil carbon losses - current c.7.3% of UK s GHG emissions (Bellamy et al, 2005) attributed to rising temperatures, but the effect of agriculture is being overlooked by researchers: (i) correlation of losses and intensity of farm management (ii) grazed grassland being classified as nonagricultural (iii) studies are measuring to a limited depth (max. 30cm) (iv) losses due to erosion excluded from some estimates so, modern farm practices may also be contributing. What comparative evidence is there on soil carbon levels? 3

4 Evidence for comparative levels Controlled long-term trials Duration, years Reference Rodale Institute FST, US 21 Hepperly et al, 2006 FiBL DOK trial, Switzerland 21 Fließbach et al, 2007; Mäder et al, 2002 IBR Darmstadt, Germany 18 Raupp and Oltmanns, 2006 Michigan University, US 9 Robertson et al, 2000 Comparisons of organic & non-organic farms 30 pairs org & non-org farms, England Armstrong Brown et al, 2000 Org & non-org tomato production, US Drinkwater et al, 1995 Org & non-org wheat field, US Reganold et al, 1987 Evidence - the findings the two US trials: organic farming builds from 80kg (Michigan, 7.5cm) to 981kgC/ha/year (Rodale, 30cm). Ascribed to the use of winter cover crops. the European trials: for same C & N rates, compost builds more SOM than inorganic fertiliser. the three farm comparisons all found organic farming produces higher SOM, for horticulture and cereal crops. this evidence is in line with farmers experiences 4

5 How much carbon can organic farming sequester in the soil? Calculating sequestration potential The carbon sequestration potential of organic farming varies considerably, depending on the scenario chosen: Example 1. If organic farming builds 320kgC/ha/year, on UK arable area of 5.9m ha 1% UK annual GHGs Example 2. If 1tC/ha/year, on global cultivated land of 1.5bn ha 47% of global annual net CO 2 emissions So, if level is high (1t/ha/yr) on all cropped land, organic farming could deliver most of the emissions targets as long as the background losses stabilised. 5

6 Assumptions about C sequestration Many negative assumptions are being used as reasons for not recognising the soil C benefits of organic farming: the use of ploughing is a weakness of organic farming higher yielding systems can build up more soil carbon the higher microbial life of OF may be a disadvantage applying high levels of organic matter has little result sequestration is not continual but mainly in early years soil carbon sequestration is non-permanent, reversible Assumptions - are they really true? No, the evidence and analysis shows they are incorrect: higher crop yields means more organic matter is being removed from the farm. In FiBL trial, the non-organic system yielded more but did not build more soil carbon. Below-ground biomass & soil microorganisms are also factors. In Rodale, despite similar above-ground C input, organic systems produced more SOM. ability to build soil C within 20 years is a real strength! 6

7 Role of agricultural policy Policy approaches The soil C benefit should be recognised by policymakers: product of organic system, no need specific practices benefit calculated in relation to the target level may not offset farm GHGs, but other policy benefits: (i) reduces drought impact & irrigation need in agriculture (ii) water-holding capacity reduces flood risk for society (iii) humus promotes the dietary nutrient supply (minerals) (iv) soil humus & microbes support biodiversity. 7

8 Surface water flooding is caused by soil saturation Conclusion Organic farming produces higher soil carbon levels than non-organic farming, as a product of the system produces higher total biomass and soil microbial levels potentially a significant benefit for mitigating climate change and also delivers other major policy benefits soil carbon should be included in all GWP assessments of organic farming, and the expansion of organic farming should be promoted for its soil carbon benefits. 8

9 The End [Thanks to those who provided the photographs] 9